In recent years, piezoelectric elements which each include a film (simply referred to as a piezoelectric film below) partially including, for example, polyvinylidene fluoride as piezoelectric resin and sandwiched between electrodes are known. For example, Japanese Patent Application Laid-Open No. H06-216422 discloses a piezoelectric element formed by disposing piezoelectric films made of polyvinylidene fluoride on opposite principal surfaces of electrodes, disposing two other electrodes on an outermost layer, and sandwiching each of the piezoelectric films between the electrodes.
Each piezoelectric film produces charges when the piezoelectric element is distorted, for example by being stretched and contracted in a predetermined direction. As each piezoelectric film produces charges, a potential difference is produced between the electrodes which sandwich each piezoelectric film. A voltage detection circuit detects the potential difference between the electrodes which sandwich each piezoelectric film thereby making it possible to detect that the piezoelectric element has been distorted. The potential difference between the electrodes which sandwich each piezoelectric film will be referred to simply as a voltage of the piezoelectric element below.
A piezoelectric resin has a relatively low dielectric constant ε (F/m) compared to that of piezoelectric ceramics and produces a low capacitance. When the piezoelectric element disclosed in the foregoing application is directly connected to a voltage detection circuit, the causes following problems to occur.
First, a voltage produced in the piezoelectric element becomes too high, and when, for example, it exceeds a drive voltage of the voltage detection circuit, it is difficult to precisely detect an output voltage from the piezoelectric element. Second, because a capacitance of the piezoelectric element is low, the voltage of the piezoelectric element is significantly influenced by noise superimposed on the charges generated by the piezoelectric film. Third, because the capacitance of the piezoelectric element is low, a time constant becomes low and the charges quickly dissipate from the piezoelectric element. When the charges quickly dissipate from the piezoelectric element, a time period during which the voltage detection circuit can detect a voltage of the piezoelectric element shortens.
It is therefore an object of the present invention to provide a piezoelectric element which increases a capacitance and includes a film partially including a piezoelectric resin and sandwiched between electrodes.